1. Field of the Invention
The present invention relates to a motor.
2. Description of the Related Art
In a so-called inner-rotor motor, in which a rotating shaft and a rotor magnet are arranged radially inside of an armature, a bending of the shaft tends to occur more easily with an increasing rotational speed of the motor. A bending of the shaft may cause vibrations and noise of the motor as a whole. In recent years, due to an increase in the rotational speed of motors, there has been a demand for preventing a bending of the shaft. A known inner-rotor motor is described, for example, in JP-A 2013-99094.
In the motor described in JP-A 2013-99094, a shaft and a rotor magnet are fixed to each other through a sleeve and a rotor yoke (see paragraph [0019] and FIG. 1 of JP-A 2013-99094). Each of the sleeve and the rotor yoke is fixed to the shaft at one axial position. With this structure, it is difficult to improve rigidity of the shaft and its vicinity, and it is also difficult to prevent a bending of the shaft while the motor is running.
An effective way to prevent a bending of the shaft is to arrange a rotor core around the shaft.
In the motor described in JP-A 2013-99094, the rotor magnet is arranged to project toward a circuit board relative to a stator core (see FIG. 1 of JP-A 2013-99094). In the case where the circuit board has mounted thereon magnetic sensors used to detect the circumferential position of the rotor magnet, it is necessary to ensure that the magnetic sensors are able to detect the circumferential position of the rotor magnet with sufficient precision. This requires some a special configuration to be used, such as reducing the distance between the circuit board and the rotor magnet as in the motor described in JP-A 2013-99094.
However, extending the rotor core up to an end portion of the rotor magnet on a side closer to the circuit board, which would not contribute to improving a torque characteristic of the motor, would result in an undesirable increased weight of the motor, resulting in a deterioration in the torque characteristic of the motor.